FIG. 1 shows an example of a conventional flash tube 1 according to prior art having external triggering. The flash tube 1 comprises a glass envelope 2 enclosing a gas 3. One electrode 4, 5 is provided at each end inside the glass envelope 2, where the electrodes 4, 5 protrudes out of the glass envelope 2 and connects to two electrical connectors 4A, 5A. The two electrical connectors 4A, 5A are arranged to receive and apply a voltage between the two electrodes 4, 5. The two electrodes 4, 5 may be charged up to a suitable voltage level, V1, using e.g. a capacitor (not shown).
The flash tube 1 illustrated in FIG. 1 is externally triggered. This is performed by having the electrodes 4, 5 of the flash tube 1 charged up to a voltage level which is high enough to respond to a triggering event, but below the flash tube's self-flash threshold. Then, a high voltage pulse, which normally may be between 2000 and 150 000 V, is externally applied directly to or close to the glass envelope of the flash tube. This may also be referred to as a “trigger pulse”. The flash tube 1 further comprises a material 8 applied to the electrode 5. This material helps to ionize the gas inside the glass envelope of the flash tube 1.
This short duration, high voltage pulse creates a rising electrostatic field, which ionizes the gas inside the glass envelope of the flash tube 1. The capacitance of the glass couples the trigger pulse into the glass envelope, where it exceeds the breakdown voltage of the gas surrounding one or both of the electrodes 4, 5, generating a plurality of spark streamers. The plurality of spark streamers will propagate randomly through the gas and via capacitance along the glass at a speed of about 1 cm in 60 ns, that is, around 170 km/s. It should be noted that a trigger pulse must have long enough duration to allow at least one of the plurality of random spark streamers generated to reach the opposite electrode, otherwise erratic triggering will occur. When at least one of the random spark streamers has bridged the electrodes, the charged-up voltage will discharge through the ionized gas, and cause a heating of the gas (e.g. xenon) to a high enough temperature for the emission of light, i.e. generate a flash.
In a camera, flash tube synchronization is defined as synchronizing the firing of the flash tube with the opening of the shutter admitting light to photographic film or image sensor. One type of flash tube synchronization is FP-sync, Flat Peak. FP-sync is used with flash tubes designed specifically for use with focal-plane shutters. A focal-plane shutter uses two shutter curtains that run horizontally or vertically across the image sensor plane. For slower shutter speeds, the first curtain opens, and after the required time with the shutter open, the second curtain closes the aperture in the same direction. Faster shutter speeds are achieved by the second curtain closing before the first one has fully opened. This results in a slit that travels across the image sensor. Faster shutter speeds simply require a narrower slit, as the speed of travel of the shutter curtains is not normally varied. Using this technique, modern SLR cameras are capable of shutter speeds of up to 1/2000, 1/4000 or even 1/8000 s.
When using a focal plane shutter, although each part of the image sensor is exposed for the rated exposure time, the image sensor is exposed by a slit which moves across the image sensor in a time, the X-sync speed. The X-sync speed may be of the order of 1/250 s. Although the exposure of each part of the image sensor may be 1/2000 s, the last part of the image sensor is exposed later by the X-sync time than the first part of the image sensor. If the flash tube discharge for a shorter time than the X-sync speed only parts of the image sensor will be illuminated. Flash tubes that discharge during the entire X-sync time will result in that the entire image sensor will be illuminated even at higher shutter speeds. When the flash tube is discharged for a long time with constant energy required to illuminate the entire image sensor, the flash tube can be considered as a fixed light source.
However, a disadvantage with a flash tube that is designed for a discharge with a much larger energy and a shorter duration is that when the flash tube is discharged at a lower energy for a long time the spark stream will start from different places on the electrode 4. This result is that each flash that is generated is usually different from each other, that is, the emitted light from one flash often comprises a different colour temperature than a subsequent flash from the same flash tube 1.
Another disadvantage with a flash tube that is designed for a discharge with a much larger energy and a shorter duration is that when the flash tube is discharged for a long time the spark stream will spread downwards on the electrode and changing the arc length during the light output. When the spark stream spreads downwards on the electrode, the material 8 will also be damaged. Small pieces of the material 8 can also come lose if the spark stream is spread down to the material 8. These small pieces of material can damage the glass envelope. Another problem associated with the damage of the material 8 is that the material will loosen its capacity to help the gas to be ionized.
There is therefore a need for an improved solution for flash tubes, which solution solves or at least mitigates at least one of the above mentioned problems.